1. Field of the Invention
The invention relates to a method for casting thin billets, in particular composed of steel, having a belt casting device, in the case of which liquid metal from a metallurgical vessel is fed via a feed device to an endless belt, and to a corresponding belt casting device.
2. Discussion of the Prior Art
When producing strips using the belt casting method with the belt surface being cooled on one side, difficulties occur in producing these strips with tight thickness tolerances.
According to German Reference DE 35 21 778 C2, it is known for metal billets to be produced in the form of strips, in which case liquid metal is applied from a nozzle onto the cooling surface with a heat sink which moves past the nozzle with a narrow gap. The nozzle is in the form of a slotted nozzle in order to produce strips. In this case, the distance between the nozzle lip on the outlet side and the cooling surface of the heat sink is adjustable. The width of the nozzle (unobstructed distance) of the nozzle lip on the billet start and billet outlet side is arranged at a distance which corresponds to the length of the cold solidification front of the strip to be cast.
Strips produced using this method and apparatus have not provided satisfactory results.
The invention is based on the aim of providing a method with a corresponding apparatus for casting thin billets, in the case of which an endless strip can be produced with tight tolerances and with the desired strip thickness using simple physical means and with reliable process control.
According to the invention, the feed rate of the liquid metal is kept constant with respect to the outlet rate of the finished product, that is to say of the cast strip which is drawn off the endless belt system. Before the liquid metal strikes the endless belt, the flow rate of the liquid metal stream is reduced to such an extent that it strikes the endless belt at a speed corresponding to the belt output speed, and the liquid metal stream has a thickness which is matched to the desired billet thickness in the impact area.
The level of the liquid metal in the metallurgical vessel upstream of the feed device is in this case set in such a manner that the geodetic height is given by P less than 10xc3x97ds, where P=level and ds=billet thickness. Both the level P and the billet thickness ds may be used as a control variable in this case.
In order to maintain the level height exactly, measurement elements are provided for detecting the level, for example a float or a bubble device, and for maintaining the strip thickness, for example a distance sensor which acts via a measurement and control device on an actuator which controls shut-off elements to regulate the flow rate. These shut-off elements may be either slides or a plug.
In order to adjust the speed of the liquid metal stream reliably, the casting channel is designed so that the first casting channel part forms a restriction channel part. The opening of the second casting channel part is designed so that it has the same cross-sectional area as the subsequent finished product.
In an advantageous refinement, the restriction channel part is in the form of a constriction element and in this case has a thickness which is less than the subsequent billet thickness, corresponding to dw=0.5xc3x97ds to 0.8xc3x97ds, where dw=thickness of the restriction channel part and ds=billet thickness.
Furthermore, the casting channel has a shape such that the first channel part, which is in the form of a restriction channel, is longer than the second channel part.
In a further advantageous embodiment, a restriction element is provided in the restriction channel, is in the form of a filter and in this case has a free area of 0.6 to 0.8xc3x97the cross-sectional area of the casting channel. The free area of this filter may in this case comprise a hole incorporated in a refractory plate.
In one embodiment, the liquid metal stream is heated in the region of the feed device. For this purpose, it is proposed that the wall of the casting channel be formed from electrically conductive refractory material, and that an induction coil be used as the heating device.
One advantageous embodiment of the invention comprises braking of the liquid metal stream. To this end, an eddy-current brake is provided on its own or in addition to the constriction described above, and its static magnetic field reduces the speed of the liquid metal stream. In a further embodiment, a linear motor is provided, which reduces the speed of the liquid metal stream by producing a moving field in the opposite direction to the flow direction of the melt.
In one particular embodiment, the first casting channel part is in the form of a tube. This tube may be arranged in the bottom of the container, pointing in the vertical direction. The mouth of this outlet tube is in this embodiment connected to a second casting channel part, which is in the form of an opening funnel whose mouth has dimensions which correspond to the finished product. In this case, the second channel part may have cross-sectional areas which become smaller in the flow direction of the melt in the casting channel part, in order to achieve a uniform outlet rate over the billet width.